The present invention relates to a method and apparatus for reducing the detrimental effects of energy generated within a field emission device ("FED") and especially as related to micropoint switches.
FED technology has recently core into favor as a technology for developing low power, flat panel displays. This technology uses an array of cold cathode emitters and cathodoluminescent phosphors for conversion of energy from an electron beam into visible light. Part of the desire to use FED technology for flat-panel displays is that such technology is conducive to producing flat screen displays having high performance, low power and light weight.
Referring to FIG. 1, a representative cross-section of a prior art FED 100 is shown generally. As is well known, FED technology operates on the principal of cathodoluminescent phosphors being exited by cold cathode field emission electrons. The general structure of a FED includes silicon substrate or baseplate 102 onto which a thin conductive structure is disposed. Silicon baseplate 102 may be a single crystal silicon layer. Alternatively, substrate or baseplate 102 may be constructed from one or more semiconductor layers or structures that include active or operable portions of semiconductor devices.
The thin conductive structure may be formed from doped polycrystalline silicon or metal that is deposited on baseplate 102 in a conventional manner. This thin conductive structure serves as the emitter electrode. The thin conductive structure is usually deposited on baseplate 102 in strips that are electrically connected. (Alternatively, the emitter electrode may be formed from the implantation of ions into baseplate 102.) In FIG. 1, a cross-section of strips 104, 106, and 108 is shown. The number of strips for a particular device will depend on the size and desired operation of the FED.
At predetermined sites on the respective emitter electrode strips, spaced apart patterns of micropoints are formed. (Micropoints are also referred to as "field emission cathodes," "field emitters" or "emitters.") In FIG. 1, micropoint 110 is shown on strip 104, micropoints 112, 114, 116, and 118 are shown on strip 106, and micropoint 120 is shown on strip 108. With regard to the patterns of micropoints on strip 106, a square pattern of 16 micropoints, which includes micropoints 112, 114, 116, and 118, may be positioned at that location. However, it is understood that one or a pattern of more than one micropoint may be located at any one site.
Preferably, each micropoint resembles a cone. The forming and sharpening of each micropoint is carried out in a known manner such as disclosed in U.S. Pat. Nos. 3,970,887, 5,372,973 and 5,391,259, each of which is hereby incorporated by reference in its entirety for all purposes. The micropoints may be constructed of a number of materials, such as single crystal silicon. Moreover, to ensure the optimal performance of the micropoints, the tips of the micropoints can be coated or treated with a low work function material.
Although not shown in FIG. 1, micropoints 110-120 are typically controlled (i.e., activated into an emitting state or deactivated into a non-emitting state) by switches (typically transistors) disposed within or proximate to baseplate 102. Examples of such switches, referred to herein as "micropoint switches," are provided in U.S. Patent Nos. 5,212,426, 5,357,172, 5,387,844 and 5,410,218, each of which is hereby incorporated by reference in its entirety for all purposes.
After forming the emitter electrode, dielectric insulating layer 122 is deposited over emitter electrode strips 104, 106, and 108, and the patterned micropoints located at predetermined sites on the strips. The insulating layer may be made from a variety of materials including silicon dioxide (SiO.sub.2), spin-on-glass or borophosphosilicate glass.
A conductive layer is disposed over insulation layer 122. This conductive layer forms extraction structure 132 which is a low potential anode used to extract electrons from the micropoints. Extraction structure 132 may be made from a variety of conductive materials including chromium, molybdenum, doped polysilicon or silicided polysilicon. Extraction structure 132 may be formed as a continuous layer or as parallel strips. If parallel strips form extraction structure 132, it is referred to as an extraction grid, and the strips are disposed perpendicular to emitter electrode strips 104, 106 and 108 thereby forming the rows of a matrix structure. Whether a continuous layer or strips are used, once either is positioned on the insulating layer, they are appropriately etched by conventional methods to surround but be spaced away from the micropoints.
At each intersection of the extraction and emitter electrode strips or at desired locations along emitter electrode steps when a continuous extraction structure is used, a micropoint or pattern of micropoints are disposed on the emitter strip. Each micropoint or pattern of micropoints serve as the cathode of the FED and illuminate one pixel of the screen display.
Once the lower portion of the FED is formed, faceplate 140 is fixed a predetermined distance above the top surface of the extraction structure 132. Typically, this distance is several hundred micrometers. This distance is maintained by spacers formed by conventional methods. Representative spacers 136 and 138 are shown in FIG. 1.
Faceplate 140 is a cathodoluminescent screen that is constructed from clear glass or other suitable material. A conductive material, such as indium tin oxide ("ITO") is disposed on the surface of the glass facing the extraction structure. ITO layer 142 serves as the anode of the FED. A high vacuum is maintained in area 134 between faceplate 140 and baseplate 102.
Black matrix 149 is disposed on this surface of the ITO layer 142 facing extraction structure 132. Black matrix 149 defines the discrete pixel areas for the screen display of the FED. Phosphor material is disposed on ITO layer 142 in the appropriate areas defined by black matrix 149. Representative phosphor material areas that define pixels are shown at 144, 146 and 148. These pixels are aligned with the openings in extraction structure 132 so that a micropoint or group of micropoints that are meant to excite phosphor material are aligned with that pixel. Zinc oxide is a suitable material for the phosphor material since it can be excited by low energy electrons.
A FED has one or more voltage sources that maintain emitter electrode strips 104, 106 and 108, extraction structure 132, and ITO layer 142 at three different potentials for proper operation of the FED. Emitter electrode strips 104, 106 and 108 are at "-" potential, extraction structure 132 is at a "+" potential, and the ITO layer 142 at a "++" potential. When such an electrical relationship is used, extraction structure 132 will pull an electron emission stream from micropoints 110, 112, 114, 116, 118 and 120. Thereafter, ITO layer 142 will attract the freed electrons.
The electron emission streams that emanate from the tips of the micropoints fan out conically from their respective tips. Some of the electrons strike the phosphors at 90.degree. to the faceplate while others strike it at various acute angles. The contrast and brightness of the screen display of the FED are optimized when the emitted electrons strike or impinge upon the phosphors at 90.degree..
The cathodoluminescent screen of a FED is typically illuminated through the use of a matrix addressable array of micropoints, as is well known to those having ordinary skill in the art. In such a configuration, the FED incorporates a column signal to activate a column switching driver and a row signal to activate a row switching driver. At the intersection of both an activated column and an activated row, a voltage differential between an extraction structure and micropoint exists sufficient to induce a field emission, thereby causing illumination of the associated phosphor of a pixel on the cathodoluminescent screen. Such matrix addressable arrays are illustrated in U.S. Pat. Nos. 5,210,472 and 5,410,218, both of which are hereby incorporated by reference in their entirety for all purposes.
The foregoing voltage differential is achieved through the use of a micropoint switch. An example of such a switch is provided in FIG. 2A, which is a reproduction of FIG. 2 of U.S. Pat. No. 5,357,172. As described below, when a micropoint switch is turned on, associated micropoint(s) are activated; i.e., kept at a sufficiently low voltage to achieve the necessary voltage differential so to induce a field emission. FIG. 2A schematically shows a micropoint assembly 200 disposed adjacent to extraction structure 132. Assembly 200 includes micropoint switches 206, 208 which are coupled to micropoints 114-118.
Referring to FIG. 2A, extraction structure 132 is continuous throughout a FED array and is maintained at a constant potential. Base electrode 202 is insulated from extraction structure 132 and is common to micropoints 114-118. Although FIG. 2A shows only three micropoints common to base electrode 202, this number is typically higher in conventional FEDs.
In order to induce field emission, base electrode 202 is grounded through a pair of series-coupled field effect transistors (FETs) 206 (Q.sub.c) and 208 (Q.sub.R) and current-regulating resistor 210(R). Resistor 210 is interposed between the source of transistor 208 and ground. Transistor 206 is gated by a column line signal S.sub.c while transistor 208 is gated by a row line signal S.sub.R. A micropoint is deactivated (i.e., placed in a non-emitting state) by turning off either or both of the series-connected transistors (206 and 208). From the moment that at least one of the transistors is turned off (i.e., the gate voltage V.sub.gs drops below the threshold voltage V.sub.t of the transistor), electrons will continue to be discharged from the micropoints corresponding to that transistor until the voltage differential between the base and the extraction structure is just below an emission threshold voltage. Conversely, when both transistor 206 and 208 are turned on (i.e., the gate voltage Vgs applied to each rises above the corresponding device threshold voltage V.sub.t), a coupled micropoint is activated resulting in a sufficient voltage differential (between the micropoint and extraction structure 132) to induce a field emission.
FIG. 2B illustrates the composite elements of FETs 206 and 208 in the form of a semiconductor MOSFET structure 220. FETs 206 and 208, in accordance with conventional MOSFET construction, include a gate 211 (made from any conventional substance such as doped polysilicon or metal) disposed over a gate oxide layer 218 made from silicon dioxide. Gate oxide 218 is disposed atop a lightly P+ doped substrate 212. As shown in FIG. 2B, a source region 214 and a drain region 216 are disposed within substrate 212 immediately beneath and to either side of gate oxide 218. These source-drain regions, as is well known in the art, may be formed through a variety of processes, including ion implantation.
Micropoint assembly 200 of FIG. 2A also includes an optional fusible link 204 which may be blown during testing if a base-to-micropoint short exists thereby isolating the micropoints coupled to electrode 202 from the rest of a FED array.
FED operation is, in some respects, similar to a conventional cathode ray tube. Electrons are emitted from a cathode and hit a phosphor covered anode to produce light. The brightness of this light depends on the emission current from cathode to anode. In order to maintain a desired brightness, the voltage difference between the anode and cathode ranges from hundreds to thousands of volts. This voltage difference creates a very large electrical field between the anode and cathode.
During FED operation, the phosphor coated anode emits energy in response to incident electrons emitted from the micropoint emitters. This energy may be referred to as "anode-based energy". Some of this anode-based energy is directed towards the baseplate of the FED and affects the operation of this underlying structure. More specifically, when a micropoint switch is constructed from a MOSFET (such as shown in FIG. 2B) disposed within or proximate to a baseplate, such anode-based energy may effectively lower the threshold voltage of the MOSFET. This lowering of the threshold voltage may cause the MOSFET, and its corresponding micropoint or micropoints, to remain erroneously activated. The relatively small distance separating the anode and cathode of a FED display makes the cathodes of such displays particularly susceptible to such anode-based energy.
Accordingly, it is desirable to provide an improved method and apparatus that reduces the effect of anode-based energy on devices typically disposed within or proximate to a baseplate in a FED cathode.